My extra strange thoughts. Sense the internet and AI does not show any SAE thread/Metric head bolts..........Header bolts are usually 12 point, maybe you tried a 3/8 6 point socket on a 12 pt head, then used a 10mm. 3/8= .375, (10mm= .393) and the 10mm 6 pt slipped over the 12 pt, jammed up, torqued and voila.... or as I said, the whole bolt is metric and you "rethreaded" the head.... or the whole bolt is metric and you have a motor with metric threads. I asked for vendor and PN so we can please see this weird bolt.
The 3/8 SAE and a 10mm thread have near the same pitch. So this adds to some concerning possibilities.
